1. Field of the Invention
This invention relates to a display device, especially to a display device with a DA converter that converts the digital image signal to an analog image signal.
2. Description of the Related Art
There has been a great demand on the market for portable devices with a display such as a portable TV and a portable telephone. All these devices need a small, lightweight and low-power consumption display device, and development efforts have been made accordingly.
FIG. 14 shows a circuit diagram of a display pixel element of a conventional liquid crystal display device (LCD). On an insulating substrate (not shown) a gate signal line 51 is disposed in one direction and a drain signal line 61 is disposed in a direction perpendicular to the gate signal line 51. A pixel element selection thin film transistor 72 connected to both signal lines 51, 61 is formed near the crossing of the signal lines, 51, 61. The thin film transistor will be referred to as the TFT hereinafter. A source 11s of the pixel element selection TFT 72 is connected to a display electrode 80 of a liquid crystal 21.
Also, a storage capacitance element 85 for holding the voltage of the display electrode 80 for one field period is formed. One terminal 86 of the storage capacitance element 85 is connected to the source 11 s of the TFT 72, and the other terminal 87 is provided with a voltage commonly used among the pixel elements.
When a scanning signal (H level) is applied to the gate signal line 51, the pixel element selection TFT 72 turns on and an analog image signal is transmitted to the display electrode 80 through the drain signal line 61 and retained in the storage capacitance element 85. The image signal voltage applied to the display electrode 80 is then applied to the liquid crystal 21. Based on the voltage applied, the liquid crystal aligns itself for providing a liquid crystal display image.
The analog image signal applied to the drain signal line 61 is obtained by converting an input digital image signal through a digital to analog conversion by a DA converter. In a conventional liquid crystal display device having a DA converter in a display panel, the DA converter is formed near driver circuits in the peripheral area of pixel elements.
However, since the DA converter is formed near the driver circuits of such a conventional device, the design flexibility of the circuits surrounding the pixel elements is restricted. This also results in a larger framing area of a display panel. Especially, when gradation is required of a display device, the number of terminals for gradation voltages increase in accordance with the depth of the display.
Also, in order to write an analog signal converted by the DA converter into a pixel element portion through the pixel element TFT 72, a voltage higher than a threshold voltage, Vth, should be supplied as the scanning signal. Here, the threshold voltage Vth is the threshold voltage of the pixel element selection TFT 72. Therefore, reducing the applied voltage and the power consumption of the liquid crystal display device is difficult.